1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a facsimile, a copying machine and the like, using an electrophotographic system, an ink jet system or a heat transfer system.
2. Related Background Art
In conventional image forming apparatuses in which an image is formed with the above-mentioned system while a recording medium such as a recording sheet is being conveyed, the following arrangement is generally known.
An apparatus shown in FIG. 6 includes an image input portion 90R and an image output portion 90P. The image input portion 90R serves to receive image information for the recording and to output an image signal to the image output portion 90P after the image information was subjected to various treatments. The image output portion 90P comprises an image forming portion 91 for forming an image, a sheet supply unit 92 for containing stacked recording sheets P and for supplying the recording sheets one by one, a transfer convey unit 93 for convey the recording sheet P supplied from the sheet supply unit 92 to the image forming portion 91 and for transferring the image onto the recording sheet, a fixing unit 94 for fixing the image recorded on the recording sheet, and a control unit 95 for controlling the entire operation of the apparatus.
Now, a recording operation will be described in order. When the operation of the apparatus is started, first of all, the recording sheet P is supplied from the sheet supply unit 92 and is sent to a pair of regist rollers 96 (now stopped) disposed in front of the convey unit 93. Thereafter, in synchronous with an image record start signal, the regist rollers 96 are rotated to seat the recording sheet P onto the convey unit 93. With this operation, an image can be formed at a predetermined position on the recording sheet.
The recording sheet P sent from the pair of regist rollers 96 is conveyed to an image transfer area T while being held on the convey unit 93. At the image transfer area T, the image from the image forming portion 91 is transferred onto the recording sheet. The holding of the recording sheet P on the convey unit 93 is generally accomplished by electrostatically absorbing the recording sheet onto a convey belt of the convey unit formed from a thin dielectric film. After the image was formed on the recording sheet P, the convey unit 93 conveys the recording sheet P to a position proximity to a fixing unit 94, and then sends the recording sheet to the fixing unit 94 while separating a tip end of the recording sheet. The recording sheet is subjected to heat (and pressure) from a fixing nip of the fixing unit 94, to thereby fix the toner image to the recording sheet. Thereafter, the recording sheet P is discharged out of the apparatus through a discharge portion.
Further, in an image forming apparatus having a plurality of image forming portions 91a, 91b, 91c and 91d as shown in FIG. 7, images formed in the image forming portions 91a to 91d by the image forming process substantially the same as described above are transferred onto the recording sheet P in a superimposed fashion at respective image transfer areas Ta, Tb, Tc and Td.
A convey drive roller 97 of the convey unit 93 is generally disposed at a downstream side of and in the vicinity of the last image transfer area. The reason is that, when the convey drive roller is disposed at the downstream side of the image transfer areas (Ta to Td), the convey belt is pulled toward a conveying direction at each image transfer area, to thereby prevent the poor image transferring and the transfer positional deviation. Further, in order to make the apparatus compact, a distance between the convey unit 93 and the fixing unit 94 should be reduced as much as possible. To this end, the convey drive roller 97 should be disposed in the vicinity of the fixing unit 94.
However, in the above-mentioned image forming apparatus, the following problems will arise.
When the power source of the apparatus is turned ON, a heater of the fixing unit 94 is automatically operated to heat the fixing unit to a predetermined temperature. A part of the heat leaks out of the fixing unit, so that the temperature of the interior of the apparatus is increased to some extent. Particularly, since the convey drive roller 97 of the convey unit 93 is disposed in the vicinity of the fixing unit 94, the temperature increase of the convey drive roller becomes greater than those of the other elements. As a result, the convey drive roller 97 is expanded to change a convey speed of the convey belt of the convey unit 93. If the change in speed of the convey belt exceeds a certain value, contraction of the transferred image will occur.
Further, in the apparatus in which the different images formed in the plurality of image forming portions are successively transferred onto the same recording sheet in a superimposed fashion as shown in FIG. 7, positional deviation between the transferred images (transfer deviation) will occur. Particularly, in the latter case, the transfer deviation occurs even when the speed is changed by 0.1% or more, depending upon a path length between the image forming portions.
In order to drive the belt of the convey unit 93 without slip, the convey drive roller 97 has generally an outer layer having high coefficient of friction (for example, rubber layer). However, in general, since such material has a great coefficient of linear expansion, the expansion of the rubber layer cannot be negligible for the image deviation. More specifically, if the temperature of the roller is increased by about 20.degree. C., a diameter of the roller will be increased by about 0.1%, and, in other words, the convey speed will be increased by about 0.1%. Further, the apparatus itself is expanded due to the increased interior temperature to increase the distance between the image forming portions, which results in the transfer deviation.
In order to solve the above problems, there has been proposed a technique in which thermal insulating material is arranged around the fixing unit to prevent heat from leaking from the fixing unit as much as possible. However, since the inlet and outlet for the recording material cannot be covered by the heat insulating material, the heat cannot be prevented from leaking through the entrance and the outlet. Further, in consideration of the cost and space for the heat insulating material, such proposal is not said to be a best solution.
Further, it is considered that the fixing heater is de-energized when the apparatus is not operated to minimize the operation of the fixing heater. In this case, however, whenever the apparatus is operated, the fixing temperature must be increased to the predetermined fixing allowable temperature (meanwhile, the operator is merely waiting to reach the stand-by condition), to thereby worsen the operability.